Within industrial, scientific and medical applications (ISM), wireless communication systems typically use signals at the carrier frequency, which is comprised within frequency ranges reserved for such applications. The carrier frequency of these signals is generally around 434 MHz or 868 MHz.
The spectral bandwidth of transmitted ASK (Amplitude Shift Keying) RF signals must, in principle, be limited to avoid interfering with the transmission of a nearby transmitter in a close frequency band and to facilitate reception of the signals by a nearby receiver. Consequently, it may be envisaged to reduce the output power, particularly in the output power amplifier of the transmitter circuit. It is also possible to envisage reducing the data transmission rate, encoding the baseband data or adapting certain modulation techniques. Controlling the transitions between two data symbols may also be envisaged.
Where a C-class output power amplifier is used for a high efficiency transmitter circuit, it is often difficult firstly to directly limit the bandwidth of the signals to be transmitted, and secondly to decrease the output power in an easy manner. A high efficiency transmitter circuit with a C-class power amplifier is not generally linear. A capacitor may, for example, be mounted between the gate and drain terminals of a MOS transistor of the power amplifier. However, this type of arrangement reduces the efficiency of the transmitter circuit. Moreover, this arrangement is generally impractical for these power amplifiers for transmitting radio frequency signals.
U.S. Pat. No. 7,560,989 may be cited, which discloses an output power amplifier circuit for the transmission of ASK RF signals as shown in FIG. 1. This transmitter circuit therefore includes an output power amplifier unit 1, which is configured to control the desired output power. The power amplifier unit 1 can be connected to an antenna assembly 4, which is shown in FIG. 1 in dashed lines from the output terminal PA—OUT of power amplifier unit 1. Power amplifier unit 1 includes several parallel amplifier cells A, B, C, D, which are generally connected between an earth terminal and output terminal PA—OUT. In this example, said amplifier cells are amplifier cells in a cascode arrangement, which are each formed of two series connected NMOS transistors N1 and N2. A determined variable current IA, IB, IC, ID can pass through each operating cell.
A power controller 2 of the transmitter circuit selects a combination of amplifier cells to be activated to set up a desired output power for transmitting ASK data signals. The selection of amplifier cells to be activated thus provides rudimentary amplitude or transmitted power control. To be able to activate this type of amplifier cell of the power amplifier unit, the gate of a second transistor N2 is controlled by a control voltage at a level close to the positive terminal of a supply voltage source. Each combination of activated amplifier cells defines a predetermined power or respectively amplitude level of the output signal from the power amplifier unit.
Said power amplifier transmitter circuit further includes (not shown in FIG. 1), a reference cell, which is a replica cell similar to one of amplifier cells A, B, C, D, a current generator and a voltage generator. The current generator and the voltage generator are connected to the power controller 2. The current generator supplies a reference current, and the voltage generator supplies a reference voltage. The reference current is mirrored in the replica cell, which is formed of two MOS transistors in a cascode arrangement. The current mirrored in the replica cell controls the current in each activated cell of power amplifier unit 1.
A voltage regulator of the power amplifier circuit can also set a regulator voltage VREG for powering an inverter, the output of which is directly connected to the gate terminal of each first transistor N1 of each cell, A, B, C, D of power amplifier unit 1. This inverter 3 receives at input a signal RF_in at a desired carrier frequency for the data signals to be transmitted. By switching on and interrupting the voltage regulator, which powers inverter 3, it is possible to obtain ON-OFF keying.
An adaptation, for modifying the power or shape of the data signals, can therefore be carried out by selecting amplifier cells A, B, C, D or also by adapting the reference current mirrored in the replica cell. However, this requires a considerable number of electronic components to achieve this adaptation, which constitutes a drawback. Moreover, there is nothing provided to reduce sufficiently the bandwidth of the ASK RF signals to be transmitted, which constitutes another drawback.
As previously mentioned, the use of amplitude shift keying (ASK) is generally at a larger bandwidth and it is often necessary to reduce said bandwidth. For this purpose, the data transition edges may be attenuated to a greater or lesser extent to reduce the effective bandwidth of the transmitted signal. To achieve this, it may be envisaged to add a third transistor in series with the other two transistors in each amplifier cell of the power amplifier unit of the transmitter circuit. This third transistor of each amplifier cell may be controlled across its gate by an ASK modulating signal to modulate the amplitude of the carrier frequency RF signals to be transmitted. The edges of each transition in the data to be transmitted are smoothed or attenuated by gradually varying the gate voltage of the third transistor. The gate voltage varies progressively on the basis of a variation in the control current in a current control loop which includes a current ramp generator. This means that the rapidity of state transition in the data to be transmitted is attenuated by the ASK RF signals.
This arrangement also requires several low-pass filters, capable of being configured by a binary word, between a replica cell of the current control loop and each amplifier cell, in order to properly adapt the shape of the ASK RF signal envelope to be transmitted. For any current ramp or envelope modification of the ASK RF signals to be transmitted, the transfer function of each low-pass filter must also be modified in the analogue domain at the power amplifier input. This filter is achieved in the usual way by an array comprising capacitors and resistors and/or one or more operational amplifiers. The filter acts on a baseband signal, which involves relatively low cutoff frequencies. These cutoff frequencies can typically be configured between several kilohertz and several hundred kilohertz. The capacitance and resistance values required are therefore relatively high, which leads to a large integration surface area. This constitutes a drawback. Adapting the data transition edges in the analogue domain also depends on the manufacturing method used. The resistance and capacitance values of the filter, and the properties of the transistors, particularly in the power amplifier, vary according to the manufacturing method. This constitutes another drawback. Moreover, several current sources of different values must be generated in the analogue domain, which constitutes another drawback.